1. Field of the Invention
The present invention relates to an active matrix display device in which thin-film transistors (hereinafter, referred to as TFTs) are provided as switching elements, and more particularly, relates to a structure of a storage capacitor in each pixel of the active matrix display device.
2. Description of the Related Art
As one type of active matrix display devices, a TFT-driving liquid crystal display device is conventionally known. The TFT-driving liquid crystal display device is widely used for OA equipment, televisions and the like because a clear image can be obtained by controlling the application of a voltage to liquid crystal for each pixel with the TFTs formed on a transparent substrate such as a glass substrate. In order to realize clearer display of characters or geometric patterns, it is required to enhance definition by reducing the size of each pixel.
FIG. 1 is an equivalent circuit diagram for one pixel of a TFT-driving liquid crystal display device. A TFT 102 is disposed at the intersection of a gate signal line 100 and a source signal line 101. A storage capacitor 104 is connected to the TFT 102 as a load of the TFT 102 in parallel with a liquid crystal capacitor 103. With this configuration, when the TFT 102 is switched ON based on a signal of the gate signal line 100, the electric potential of the source signal line 101 is written to a pixel electrode portion 105, thereby accumulating electric charges in the liquid crystal capacitor 103 and the storage capacitor 104. When the TFT 102 is switched OFF, only the electric charges accumulated in the liquid crystal capacitor 103 are stored. By disposing the storage capacitor 104 in parallel with the liquid crystal capacitor 103 as shown, however, a storage property can be improved.
The storage capacitor 104 has a function of restricting a shift in the voltage of a display electrode occurring on the operation of the TFT 102. Namely, in the overlapping portion between the gate signal line 100 and the source signal line 101, a change in a parasitic capacitor 107 occurs in accordance with the ON/OFF operation of the TFT 102. Accordingly, the storage capacitor 104 is disposed in parallel with the liquid crystal capacitor 103 to increase the total amount of capacitance, thereby reducing the effects of direct-current components on the electric potential of the pixel electrode portion 105 due to the parasitic capacitor 107.
For the above advantage, the storage capacitor 104 is considered as an essential circuit component for the pixel of the TFT-driving liquid crystal display device.
FIG. 2 shows an exemplary conventional storage capacitor. An active layer 201 formed on a glass substrate 200 and a capacitor wiring 203 formed of the same film as that of a gate wiring, each serving as an electrode, interpose therebetween a dielectric 202 formed of the same film as that of a gate insulating film to form the storage capacitor. This structure is advantageous in that a highly reliable and high-quality storage capacitor can be formed even at a small thickness by using the gate insulating film as the dielectric 202.
However, the amount of the storage capacitor required for one pixel is determined based on the ratio of a channel width to a channel length of the TFT serving as a switching element, the amount of the parasitic capacitor and the like. Then, the area of the capacitor element is determined from a capacitor value for a unit area of the dielectric. Therefore, a necessary capacitor value is substantially satisfied by controlling the area of the dielectric 202 in accordance with a necessary capacitor value.
In general, the storage capacitor is conventionally formed outside the pixel TFT area, that is, in the display area. Therefore, the area occupied by the capacitor element for sufficiently reserving the capacitor value increases in the pixel area to adversely decrease an aperture ratio of the pixel, a light transmittance, and the contrast. As a result, there arises a problem that a clear screen display cannot be obtained. In particular, when it is desired to realize a high-definition display device, this drawback becomes noticeable.
The first object of the present invention is to obtain the structure of a capacitor element which allows a sufficient amount of the storage capacitance to be reserved even when the area for one pixel is reduced in order to realize high definition of the active matrix display device.
The second object of the present invention is to improve the display retention property of each pixel without decreasing the aperture ratio in the active matrix display device.
The third object of the present invention is to obtain the structure of a capacitor element with high reliability in the active matrix display device.
By achieving the above first, second, and third objects, the present invention achieves another object of fabricating a display device with a high aperture ratio and a sufficient amount of the storage capacitance.
Furthermore, the present invention has the object to improve a throughput and reliability of a display device (typically, a liquid crystal display device or an electro luminescence (EL) display device) in which TFTs are integrated on the same substrate. On the other hand, the present invention has a further object to improve the reliability of electric appliances that utilize the display devices fabricated by using the present invention.
In order to achieve the above first object, first it is necessary not to form a capacitor element in the display area which affects the aperture ratio. Therefore, the inventor of the present invention conceives to form the capacitor element in the pixel TFT area. Furthermore, in order to obtain a sufficient amount of the storage capacitance without increasing the pixel TFT area, the object can be achieved by disposing the capacitor element below the pixel TFT area as shown in FIG. 3.
In order to achieve the above second object, in addition to meeting the above first requirement, one or more storage capacitors may be provided. FIG. 4 shows an example where two storage capacitors using a first wiring 301, a second wiring 303, and a third wiring 314 are provided below the pixel TFT area. With this structure, a sufficient area can be reserved even when the area for each pixel is reduced to achieve the display with high definition. As a result, a good storage property can be obtained.
In order to achieve the above third object, in addition to meeting the first and second requirements, it is necessary to dispose the capacitor element below the pixel TFT area. By disposing the capacitor element below the pixel TFT area, a highly reliable film can be obtained because a sufficient thermal treatment for improving the film quality can be conducted on the dielectric.
Moreover, by using a material having a light-shielding property for the capacitor wirings, the deterioration of the active layer (semiconductor film) due to incident light can be reduced.